1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and a facsimile.
2. Description of the Related Art
A conventional image forming apparatus has been proposed in Japanese Patent Laid-Open No. H09-50187. This image forming apparatus employs the counter developing system in which the rotational direction of the photosensitive drum and the rotational direction of the developing roller opposed to the photosensitive drum are opposite in the developing portion.
Also, a conventional developing roller has been proposed in Japanese Patent Laid-Open No. 2000-321864. In this developing roller, a plurality of linear grooves extending in the axial direction perpendicular to the driving direction on the sleeve surface are formed at a predetermined interval in the circumferential direction of the developing sleeve so that the developer borne on the developing sleeve of the developing roller can be sufficiently conveyed.
However, in the counter development method of Japanese Patent Laid-Open No. H09-50187, the photosensitive drum rotates in the direction opposite to the conveying direction of the developer borne on the developing roller at the developing portion. Thus, the magnetic brush which is in contact with the photosensitive drum receives a force in the direction opposite to the developer conveying direction. In particular, the closest portion of the photosensitive drum and the developing roller is made narrow in order to obtain high developing performance. Thus, the magnet brush is likely to receive an opposing force. As a result, the developer borne on the developing roller becomes difficult to pass through the closest portion and the developer is likely to remain in the vicinity of the closest portion.
When stagnation of the developer occurs at the developing portion, toner which has been used for developing an electrostatic latent image at an upstream portion of the rotational direction of the photosensitive drum with respect to the developing portion on the photosensitive drum is scraped by the developer stagnating at a downstream portion of the rotational direction of the photosensitive drum. As a result, image defects such as image blurring occur.
When the stagnation of the developer has deteriorated, the developer including magnetic carrier coated as a thin layer and borne on the developing sleeve by the magnet in the developing sleeve moves to a position where retention by a magnetic force does not work due to the stagnation. The developer including the magnetic carrier overflows from the developing device thereby a defect occurs such as discharging in a state where the magnetic carrier is attached to the sheet. Furthermore, the magnetic carrier which has overflowed from the developing device is conveyed to the transfer means, the fixing means and the like as well as the toner image. As a result, a large part of the main body of the image forming apparatus is damaged.
Therefore, in the counter developing system, a high conveying performance of the developer is required at the closest portion between the photosensitive drum and the developing roller. However, even with the developing sleeve to form linear grooves of Japanese Patent Laid-Open No. 2000-321864, conveying performance deteriorates and the stagnation is likely to occur.